Ive had both the 60mm and the 105mm recently, and both are fantastic lenses. I ended up settling with the 60 as I preferred the focal length.

Here's a thought though. If you want to do serious macro work and get beyond 1:1, you will need to get a non-G lens like the 200mm or older 60 or 105mm lenses. Then you will be able to use the cheap extension tubes to get diaphragm coupling. I believe the latest Kenko tubes will couple with gelded (G) lenses though but I may be wrong here.

It is easy to manual focus (in most cases I assume better than AF), just something you need to practice, and most of all, use of a tripod is recommended.
If you are serious about macro I would go for either the 105 or 60mm Nikkors depending what you will shoot. And yes FX lenses work on DX bodies and v.v., but DX lenses on FX bodies use the same size of the sensor as a DX sensor.

Or would it be better to just buy the Kenko tubes and use my Kit Lens and lose some feutures the Macro Lens brings along?

I guess i could buy a Macro Lens and the Kenko tubes and fit them both togeather to get a huge Macro, right? And one of my goals is to take a close up picture of a snow flake and possibly work on that for a while, want to experiment.

Edit: Oh and Yes i already have a very good Tripod by Manfrotto. Infact, it's the same as yours

But Jeremy, one thing i do not unterstand is, why the 60mm Lens when the 105mm Lens has more focal length. Don't you want to be as far away from a objekt as possible? Like if it's a Bee, it would fly away getting too close.

I have the old 60mm nikkor macro and it is amazing, I really like the 60mm focal length. Focusing speed shouldn't be a concern as im sure you will discover manual focus is the best way to go.
I think the focal length really depends on what you are going to be shooting most, 100-150mm would be better for insects and other moving subjects, with non moving subjects, focal length is not so important.

Also remember that with the longer focal length macros, you will probably need image stabilization (or not if always using a tripod)

I personally think that it would be best to look at long focal length, not only will you not scare the subject away (If you're photographing subjects such as insects or arachnids) but it won't cast a shadow on the subject either. Frankly, I'd want to have the longest macro focal length possible, although that would also be the highest cost possible! Fortunately, macro lenses generally have VR, so less effect from camera shake.

I'd go for the 105mm, and it's also worth noting that as it's an FX lens, your FoV will be even larger as the D90 is a DX body, which is a bonus! However it's your call, all the Nikon Micro lenses are fine choices.